Patent application title: Method and arrangement of measuring acidity for other chemical or physical property of a gas

Abstract:

A method and an arrangement of measuring acidity or other chemical or
physical property of a gas. The invention comprises a membrane having
optical indicator molecules bound to a microporous matrix arranged to be
placed into contact with the gas to be measured, the optical indicator
molecules changing their colour in response to the acidity or other
chemical or physical property of the gas, a light source, and a detector.
The light source is arranged to emit and direct light to the membrane,
the light being transmitted through the membrane, whereby a part of the
light is absorbed into the optical indicator molecules, and the rest of
the light emitted is guided to the detector, where it is measured.

Claims:

1. Method of measuring acidity or other chemical or physical property of a
gas, the method comprising the following steps,arranging a membrane
having optical indicator molecules bound to a microporous matrix is
placed into contact with the gas to be measured, the optical indicator
molecules changing their colour in response to the acidity or other
chemical or physical property of the gas, anddirecting light to the
membrane, whereby a part of the light is absorbed into the optical
indicator molecules, and measuring the light passed through the membrane.

2. The method according to claim 1, wherein monochromatic light is used.

3. The method according to claim 1, wherein light having more than one
wave lengths are used.

4. Arrangement at measurement of acidity or other chemical or physical
property of a gas, the arrangement comprising,a membrane having optical
indicator molecules bound to a microporous matrix arranged to be placed
into contact with the gas to be measured, the optical indicator molecules
changing their colour in response to the acidity or other chemical or
physical property of the gas,a light source, anda detector,the light
source being arranged to emit and direct light to the membrane, the light
being transmitted through the membrane, whereby a part of the light is
absorbed into the optical indicator molecules, and the rest of the light
emitted is guided to the detector, where it is measured.

5. The arrangement according to claim 4, wherein the light source is
arranged to emit monochromatic light.

6. The arrangement according to claim 4, wherein the light source is
arranged to emit more than one wavelength.

7. The arrangement according to claim 4, wherein the membrane having
optical indicator molecules bound to the microporous matrix is a dye film
deposited on a substrate.

8. The arrangement according to claim 7, wherein the membrane having
optical indicator molecules bound to microporous matrix is a dye film
fabricated by the Sol-Gel method.

9. The arrangement according to claim 7, wherein the dye film deposited on
the substrate is covered by a porous mirror formed by at least one layer
allowing ions to pass through and reflecting light backwards to the
detector.

10. The arrangement according to claim 9, wherein the porous mirror is a
dielectric mirror.

Description:

[0001]The invention relates to a method and an arrangement of measuring
acidity or other chemical or physical property of a gas.

[0002]There are several well-known and widely used methods to measure the
acidity (pH value) of a liquid. pH measurement is one of the most
important measurements in laboratories and chemical industry, and its
applications range from heavy industry to microanalytical measurements in
biology. Numerous pH measurement instruments are commercially available
using glass electrode sensors or semiconductor-based ISFET sensors.

[0003]While the liquid acidity measurement technology is well-established,
there is also a need to measure the acidity of gases. An example of this
use is the measurement of acidity of flue gases, where sulfur dioxide is
often present. When sulfur dioxide is dissolved into water, sulfuric acid
results.

[0004]There are no known methods to measure the acidity directly from a
gas. Instead, the gas is in the prior art first dissolved into water, and
then the pH value of the water is measured. If the amounts of water and
gas are known, and the extent to which the gas dissolves into water is
known, the acidity of the gas can be determined.

[0005]In practice, this method is error-prone in many applications. With
the flue gas application the gas may be fed through water in a wet gas
scrubber. The acidity of the scrubber water is then monitored. If there
is an emission peak, the scrubber will not be able to dissolve all
acidity from the flue gas, and the acidity of the water does not give a
true image of the acidity of the gas.

[0006]The electrochemical pH sensors commonly used in aqueous applications
cannot be used in gas applications, as these sensors require the medium
to be conductive and gases are practically complete electric insulators.
Also, the measurement principle of these sensors requires a thin liquid
layer to be present on their surface.

[0007]The object of the invention is to create a simple method and
arrangement by which acidity or other chemical or physical property of a
gas can be measured. This is obtained with the invention. The basic idea
in the invention is to use optical indicator molecules, which change
their colour in response, for example to the acidity of the environment.
In this case the colour of each indicator molecule depends on the
molecular-level interactions it has with the molecules in its immediate
vicinity. If indicator molecules are exposed to a gas, their colour is
determined by the acidity of the said gas. The method of the invention is
characterized in that the method comprises the following steps, arranging
a membrane having optical indicator molecules bound to a microporous
matrix is placed into contact with the gas to be measured, the optical
indicator molecules changing their colour in response to the acidity or
other chemical or physical property of the gas, and directing light to
the membrane, whereby a part of the light is absorbed into the optical
indicator molecules, and measuring the light passed through the membrane.
The arrangement of the invention is characterized in that the arrangement
comprises, a membrane having optical indicator molecules bound to a
microporous matrix arranged to be placed into contact with the gas to be
measured, the optical indicator molecules changing their colour in
response to the acidity or other chemical or physical property of the
gas, a light source, and a detector, the light source being arranged to
emit and direct light to the membrane, the light being transmitted
through the membrane, whereby a part of the light is absorbed into the
optical indicator molecules, and the rest of the light emitted is guided
to the detector, where it is measured.

[0008]The main advantage of the invention is its simplicity, which makes
the introduction and use of the invention advantageous. Another advantage
of the invention is that the measurement can be automated in an
advantageous manner.

[0009]In the following the invention will be described by means of
embodiments shown in the attached drawing, whereby

[0010]FIG. 1 shows a first embodiment of the arrangement of the invention,

[0011]FIG. 2 shows a second embodiment of the arrangement of the
invention,

[0012]FIG. 3 shows a third embodiment of the arrangement of the invention,
and

[0013]FIG. 4 shows a fourth embodiment of the arrangement of the
invention.

[0014]The basic thing in the present invention is to use optical
indicators, i.e. indicator dyes. As told above one of the advantages of
the invention is its simplicity. Despite the simplicity of the basic
principle, there are practical challenges associated with this approach.
Most indicator dyes are crystalline solids in their pure form. As only
the surface of the crystal may interact with the atmosphere surrounding
it, only a small fraction of the molecules exhibit a colour change. This
can be changed by, e.g., grinding the crystals to a very fine powder, but
keeping this powder thinly spread and stationary in a flow of gas is very
difficult.

[0015]If how ver the indicator molecules are bound to a microporous
membrane matrix, the resulting structure fulfills the requirement of
large exposed surface area and yet keeps the molecules stationary. The
microporous membrane can advantageously be deposited on a substrate. This
basic thought is used in the invention.

[0016]There are several methods for making such microporous structures,
the most well-known being the Sol-Gel method. In the Sol-Gel method glass
is manufactured at low temperatures by using a polymerization reaction.
This method allows for tailoring the porosity and pore size of the
membrane, and it is possible to use different chemical compositions of
the membrane. The Sol-Gel method is described in more detail for instance
in the book Sol-Gel Science, The Physics and Chemistry of Sol-Gel
Processing, Academic Press, Inc. 1990. A Sol-Gel solution, i.e. a sol, is
a colloid solution, which forms an inorganic polymer, glass, when drying
on a glass surface. The glass sheet is coated for instance by immersing
the sheet into the sol. In this connection a reference is made also to
U.S. Pat. No. 6,208,423 B1 describing principally the Sol-Gel method.

[0017]Thus, a sensor capable of direct measurement of gas acidity can be
constructed by using indicator dye bound into a porous membrane.

[0018]In FIG. 1, a gas acidity measurement sensor is shown. The
construction has a light source 1, a flow cell 2 having the gas to be
measured inside, two optical windows to the flow cell 3, 4, and a
detector 5. One of the process windows 3 is coated with a microporous
acidity indicator membrane or film 6, i.e. the window 3 acts as a
substrate on which the microporous acidity indicator membrane or film 6
is deposited.

[0019]Light 7 emitted from the light source 1 goes through the window 3
essentially unchanged. After passing through the first window, the light
encounters the microporous membrane or film 6, which is deposited on the
window and carries the indicator dye. There are several possible
indicator dyes, e.g., bromocresol purple, bromothymol blue, thymol blue.
As the light passes through this window, some of the light is absorbed
into the indicator dye. The amount of light absorbed depends on the dye,
the wavelength of the light, and the acidity of the gas flowing through
the flow cell. The light, which has passed through the indicator formed
by membrane or film 6 is transmitted through the second window 4 and
falls on the detector 5, where it is measured.

[0020]Persons skilled in the art will recognize that the light source 1
and detector 5 may be constructed in several different ways. The light
source may be monochromatic, emitting a single wavelength. If this is the
case, the detector may be a simple light detector, e.g., a photodiode. It
is also possible to use a light source with more than one wavelengths,
e.g., several LED light sources optically combined. Thus it becomes
possible to measure the absorption of the film on several wavelengths.
Yet another variation is to use a broadband light source emitting several
wavelengths at a time, e.g., a white incandescent bulb, and to use a
colour detector instead of a simple light detector.

[0021]FIG. 2 illustrates a further alternative embodiment of a gas acidity
measurement sensor. The second window 4 shown in FIG. 1 is replaced by a
mirror 9, and the detector 5 is moved close to the light source. The
advantage of this embodiment is twofold; the light 7 passes twice through
the indicator 6, which doubles the signal, and there is no need for a
second process window. This enables installing the instrument in a larger
pipe or any other volume with gas 8.

[0022]Turning now to FIG. 3, the advantages of the previously mentioned
embodiment can be achieved by using a window 3 with integrated porous
mirror 10, i.e. this embodiment uses a porous mirror 10 disposed on the
membrane or film 6. The integrated porous mirror 10 is for example a
dielectric mirror comprising one or several porous layers. The dielectric
mirror can be manufactured for instance of two materials having very
differing refractive indexes. The film structure, i.e. pack film, can
preferably be made to a multi-player structure, whereby the pack film
should have at least three layers. It is especially preferable to form a
pack in such way that it alternatively comprises a layer having a high
refractive index and a layer having a low refractive index. A single
layer of titanium oxide alone reflects about 20% of the incident light,
but the reflection gets essentially better when layers are added. A
five-layer pack provides a reflection of about 70% already. This mirror
structure is described in U.S. Pat. No. 6,208,423 B1. This mirror
structure allows the gas molecules to pass to the indicator film 6 while
reflecting the light 7 backwards to the detector 5.

[0023]FIG. 4 illustrates a further alternative embodiment. The light
source and detector are separated from the measurement sensor by using
optical fibres 11 to carry the light. This makes it possible to move the
light source 1 and detector 5 away from the measurement point. This may
be desirable to, e.g., improve safety or otherwise remove the
optoelectronic components from the vicinity of the gas.

[0024]In this embodiment, another change is introduced; the transparent
plate 12 carrying the indicator layers 6 is immersed in the gas. In this
construction it is possible to use an indicator film on both sides of the
indicator plate. A further alternative is to use a solid piece of porous
material with embedded indicator.

[0025]In all embodiments additional optical components, e.g., lenses,
mirrors, image conduits, prisms, may be used to steer the light.

[0026]The embodiments of the invention described above are by no means
intended to restrict the invention, but the invention can be modified
completely freely within the scope of the claims. It is thus clear that
for example the arrangement of the invention or its details do not
necessarily need to be just like shown in the figures, but solutions of
another kind are also possible. In other words, although the embodiments
shown in the figures relate to several possible constructions for a gas
acidity measurement sensor, various modifications and combinations of the
embodiments as well as other embodiments of the invention will be
apparent to those skilled in the art. It should be especially noted that
by altering the indicator dye used in the invention, the sensor may be
modified to measure oxygen, redox potential, or other chemical or
physical property, which can be measured by colour indicators.